To correct spinal deformities caused by either injury or defects in the natural formation of the spine, a spinal fixation system is typically employed in an osteosynthesis surgical procedure. A typical spinal fixation system may incorporate a plurality of bone fasteners substantially aligned along the length of the spine each interconnected by a spinal rod. The spinal rod is received in a channel formed in the head of the bone fasteners. The bone fastener may be anchored to the lamina or pedicle of the vertebral body by either a threaded shank or hook which extends from the head of the fastener. The hook-type bone fastener may, in addition, be anchored to the transverse process.
Surgeons have, however, often encountered considerable difficulty when performing this surgical procedure, due to problems associated with aligning the spinal rod(s) within the rod receiving channels formed in the heads of the bone fasteners. For example, the heads of the bone fasteners may be out of vertical and/or horizontal alignment with one another due to the curvature of the spine or the size and shape of each vertebrae. This misalignment may require that the spinal rod be bent so that it may be properly seated within the rod receiving channel. The spinal rod may additionally be bent to provide a selected configuration for correction of the spinal defect. The spinal rod may also have a straight configuration. The forced interconnection of the bone fastener via the spinal rod may transfer corrective stresses to the patient's spine.
One type of bone fastener often used in the spinal fixation system is referred to as a top loading bone fastener. More specifically, the bone fastener may include a head having a U-shaped rod-receiving channel extending therethrough and a top opening to receive a closure cap to secure the spinal rod in the U-shaped channel. The U-shaped channel communicates with the top of the head of the bone fastener and defines a passageway for receipt of the spinal rod. The head of the bone fastener itself is typically spherical or cylindrical in nature in which the external side surfaces of the wall may be arcuate in shape, although other shapes are possible.
Instruments for reducing, i.e., moving, the spinal rod into the U-shaped channel of the bone fastener are typically necessary. Often times, more than one surgical instrument is used wherein one instrument attaches to the bone fastener to either stabilize the head of the bone fastener during the rod installation procedure or to act as a guide for a second surgical instrument which introduces the spinal rod into the U-shaped rod-receiving channel. A problem with an installation procedure requiring two surgical instruments is that it does not enable a surgeon to maintain one hand free, possibly requiring multiple surgeons to complete the surgical procedure. Alternatively, when a single surgical instrument is used to provide both the stabilization of the implant and application of the necessary introductory force to the spinal rod, the surgical instrument typically engages the head of the bone fastener on both sides of the U-shaped channel and thus at least a portion of the surgical instrument straddles across the U-shaped channel. That is, generally speaking, the surgical instrument attaches in a manner that encroaches over the top opening of the U-shaped channel, severely limiting access to the open channel. In addition, a rod reducing instrument which engages the head of the bone fastener on both sides of the U-shaped channel is limited in its angle of approach with respect to the bone fastener and therein unnecessary complicates the surgical procedure especially where the geometry of the surgical area does not allow the surgeon to grasp both lateral sides of the head and install the closure cap. The surgical area may be decreased and be smaller because of the natural anatomy of the patient and by the surgical instrument that must be secured to both lateral sides of the U-shaped channel formed in the head of the bone fastener.